This application is based on Japanese Patent Application No. 2002-101434 filed on Apr. 3, 2002, the disclosure of which is incorporated herein by reference.
The present invention relates to a starter used to start an internal combustion engine. More specifically, the present invention relates to a magnet switch for the starter.
There are various efforts to cope with recent environmental issues in a field of automobiles. With this, a vehicle starter has required various improvements, such as compactness, light weighting and long wearing against an engine idle stop (for eco-run). To meet this requirement, a starter disclosed in JP-A-9-68142 (U.S. Pat. No. 5,525,947) reduces damage to a ring gear by improving meshing of a pinion. Further, the starter is capable of a fine electronic control by reducing a switch electric current. That is, an electric power is supplied to a sub contact through a resistor so that a motor starts rotation at a low speed and the pinion is brought into mesh with the ring gear with that rotation force. Thereafter, the electric power is supplied to a main contact so that the motor rotates at full speed. Therefore, only an attraction force to close the contact for the motor is required. Accordingly, the switch electric current is reduced more than seventy percents as compared with the case that the pinion is meshed by the force of the switch. Thus, it makes possible to turn on and off the switch electric current by using semiconductor, reduce the size.
In the above-described starter, damage to the ring gear is reduced. Also, wear of the main contact is reduced by the virtue of the sub contact. However, problems arise from the sub contact.
For example, in a point of a durability of the sub contact, because the sub contact is made of metal and is on and off an electric current of about 100 A, it is easily worn due to arc heat. In another aspect, because a resistor is required, durability of the resistor becomes a problem. For example, if a key switch of the vehicle is locked at a starter position, the electric current may be continuously supplied to a coil. As a result, the components, such as the resistor and a coil, are burned and the sub contact cannot perform properly.
In further another aspect, chattering of the sub contact or an increase in a minimum operation voltage becomes problem. FIG. 6 is a graph showing relationship between a force F and a stroke D of the switch. In FIG. 6, a force F1 that is required to operate the switch and an attraction force F2 generated by the switch are plotted. The force F1 is determined by converting loads of a return spring, a contact pressure spring and the like driven by the switch to the force of the plunger.
When the plunger is moved, the sub contact is closed at a point Da. When the motor current is supplied through the sub contact, the voltage drops, thereby reducing the switch current. In a case that a power supply voltage is high enough, the switch continuously attracts the plunger. However, in a case that the power supply voltage is low, the attraction force F2 reduces lower than the force F1, as denoted by dotted line in FIG. 6. Therefore, the plunger is returned back with the spring force, thereby opening the sub contact. As a result, the sub contact chatters at the point Da. At the worst, the contact may be excessively heated and stuck together. The above mentioned minimum operation voltage is the minimum voltage required to avoid chattering.
As it may be understood from the above description, the sub contact causes a rapid voltage drop in a middle of the stroke of the plunger. Therefore, the minimum operation voltage of the switch having the sub contact is higher than that of a switch without having the sub contact. When the main contact is closed, the voltage drop is caused. However, when the main contact is closed, the attraction force of the switch is high enough because the plunger has been moved enough. Therefore, it does not cause the problem to the main contact. To solve the problems to the sub contact, the switch, contact and resistor may be enlarged. However, it opposes compactness.
As a means to solve the wear and sticking of the contact, brushes are used as the contact for the motor in publication JP-A-9-310666 (U.S. Pat. No. 6,054,777). However, since the brushes are wearing parts, a predetermined length is required as for a wearing margin. In the starter, the brushes generally have the wearing margin about 10 mm.
Further, the brush generally requires 10 N to 20 N as a set force. In general, two brushes are provided. Therefore, 20 N to 40N are required in total. To provide this force by a spring, it is required to bend the spring about 10 mm to 20 mm (at least about 10 mm). In the case that the brushes are used as the contact, to cope with the change in the length about 10 mm to 20 mm due to wearing of brushes and the setting force, more than 20 mm is required as a stroke of the switch and more than 20 N is required as the attraction force to move the brushes with the plunger.
Actually, it is difficult to meet both the requirements with the switch of the general starter because of its size. There is a trial to increase the stroke and the attraction force by using a principle of a lever (for example, disclosed in DE10018467 A1). However, it is difficult because a working load of the stroke by the attraction force, which cannot meet as the switch, cannot satisfy the working load even if the ratio is changed by the principle of lever. Further, an additional, special, large brush moving means is required.
That is, in the case that the brushes are used as the contact, a large-scale mechanism, such as a large switch, is required. However, this opposes the purpose to reduce the size. Also, the brushes of the starter are generally made of graphite including 50 percents or more copper to reduce contact resistances. Therefore, the brushes are not preferable as the contact.
The present invention is made in view of the above disadvantages, and it is an object of the present invention to provide a magnet switch suppressing wear and sticking of contacts due to arc by proving a sub contact from a carbon material and by turning on and off an electric power supply in a condition that an electric current is substantially zero.
According to one aspect of the present invention, a magnet switch for a starter includes a first contact portion and a second contact portion, which operate electrical connection between a motor and a battery with a fixed contact and a movable contact. The first contact portion includes a carbon material. The first contact portion and the second contact portion are disposed such that the first contact portion makes contact before the second contact portion makes contact.
Because the second contact portion is not directly conducted, damage to the second contact portion, which has metal contacts, is suppressed. Since the first contact portion includes the carbon material that does not easily stick and has lubrication, even if arc occurs in the first contact portion due to thrash of the contact when the electric power is supplied to the motor, it is less likely that the contact will stick or abnormally wear.
According to a second aspect of the present invention, when the magnet switch is turned off, the first contact portion breaks contact after the second contact portion breaks contact. Since the electric current is not directly cut off in the second contact portion, it is less likely that arc will occur in the second contact portion, thereby suppressing damage to the contact. Since the first contact portion, the electric current to which is directly cut off, includes the carbon material having strength against the arc, it improves performance of the switch.
According to a third aspect of the present invention, a resistive component is included in a first circuit in series with the first contact portion. Because the electric current is restricted in the first circuit, load to the contact can be reduced. Further, the first contact portion and the second contact portion are connected in parallel. When the second contact portion makes contact, the electric power is not generally supplied to the first circuit including the resistive component, but supplied to the second circuit. Therefore, a necessary amount of the electric current is supplied to the motor.
According to a fourth aspect of the present invention, the carbon material is made of a hundred percents carbon or made of carbon with ten percents or less metal content. Thus, the carbon material provides a resistance (generally 50 milliohm). Because the resistive component is provided by the carbon material, the number of parts is reduced and structure becomes simple.
According to a fifth aspect of the present invention, a magnet switch includes an attraction coil generating an attraction force when excited, and a contact unit for electrically connecting a battery and a motor. The contact unit includes a first contact portion for auxiliary supplying electric power to the motor through a resistive component and a second contact portion for mainly supplying electric power to the motor. The resistive component is provided by a contact of the first contact portion made of a carbon material.
When the attraction coil is excited, a movable contact and a fixed contact of the first contact portion are brought into contact with each other while bending a return spring by the plunger and then a movable contact and a fixed contact are brought into contact with each other by further movement of the plunger. When the power supply to the motor is turned off, the movable contact and the fixed contact of the second contact portion are separated before the movable contact and the fixed contact of the first contact portion are separated, by the plunger moved with a return force of the return spring. Accordingly, on and off of the switch is controlled with a simple structure.
According to a sixth aspect of the present invention, the magnet switch further includes a resilient member for applying contact pressure to the first and the second contact portions. Therefore, it suppresses a voltage drop at the contact portions. Further, even if the contact is worn, the contact portion can make contact.
According to a seventh aspect of the present invention, the fixed contacts of the first and second contact portions are provided to be connected to the battery. Therefore, the contacts can be fixed to a starter body, thereby improving reliability. Further, the movable contacts and the resilient member, which are movable, are provided adjacent to the plunger. Therefore, this makes structure simple. For example, the movable contacts and the resilient member can be commonly used.
According to an eighth aspect of the present invention, a contact pressure to the first contact portion is gradually increased or decreased by a resiliency of a resilient portion in accordance with movement of the plunger. Therefore, the resistance of the contact provided by the carbon material is gradually changed from an infinite to a predetermined value (for example, 50 milliohm). In accordance with this, the electric current is gradually changed (for example, from 0 A to 100 A and 100 A to 0 A). Because the first contact makes contact and breaks contact in the condition that the electric current is zero, damage to the contact is decreased.
According to a ninth aspect of the present invention, the carbon material is formed into two layers. The first contact portion makes contact via the layer made of one hundred percents of carbon first, and breaks contact via the layer lastly. Therefore, it suppresses arc at the first contact portion and sticking of the contacts.
According to a tenth aspect of the present invention, the motor is rotated at full speed after a pinion is meshed with a ring gear at a low speed by the electric power reduced by the first contact portion. Therefore, damage to the ring gear decreases. Further, the first contact portion makes contact via the carbon material in a condition that the electric current is substantially zero. Therefore, a life of the switch increases.